Semiconductor lasers in optical disk drives are used to read information from optical disks and, in the case of rewritable optical disks including phase-change optical disks, known as CD-RW, to write information to such optical disks.
The output level of laser light of such semiconductor lasers is switched depending on whether to write, read, or erase information on optical disks. For example, when reading information, a semiconductor laser irradiates an optical disk with laser light at an output level lower than that when writing information. The semiconductor laser thus reads information without destructing a written pit on the optical disk.
Laser light used to irradiate an optical disk to read information from the optical disk is reflected from the optical disk into a photo-electric converter. At the same time, part of the reflected light is incident on the semiconductor laser serving as the light source. As a result, this feedback light and the irradiating light interfere with each other to generate scoop noise and mode hopping noise. This causes degradation of the C/N ratio of a read signal.
A known drive circuit for a semiconductor laser uses a high frequency (HF) superimposing method to superimpose an HF current at 200 MHz to 600 MHz on a drive current for the semiconductor laser, thus reducing noise.
When writing information to an optical disk, a method is used in which an output of the semiconductor laser is subjected to pulse-width modulation (PWM) and then to intensity modulation to increase the density of information written to the optical disk and to increase the information transfer rate. In this method, the intensity of the laser light must be switched between a plurality of levels. Higher the transfer rate, shorter the minimum pulse width of the laser light (such as a few nanoseconds).
To satisfy these requirements, it is essential that the response speed of the semiconductor laser drive circuit be high. In other words, a broadband semiconductor laser drive circuit is necessary.
A semiconductor laser drive circuit for use in a known phase-change optical disk or the like tends to reduce the capacitance parasitic on the outputs of pulse current sources in the case of a method of adding the pulse current sources at an input terminal of the semiconductor laser or to narrow the bandwidth in the case of the HF superimposing method for reducing scoop noise caused by feedback laser light by a coupling capacitor disposed between the input terminal of the semiconductor laser and the output of an HF oscillator. As a result, the response speed tends to become slower. Since a laser driver is integrated with a waveform generator, many signals must be supplied to control the driving of the semiconductor laser. Upon transmission of a plurality of pulse signals, these pulse signals go out of timing due to differences in wire length. This problem is very difficult to alleviate. The circuit configuration is more complicated than that of a circuit including only the drive, and the number of elements is thus increased. Therefore, the semiconductor laser drive circuit has large power consumption and is disadvantageous in terms of heat dissipation.
In order to solve the above-described problems of the related art, it is an object of the present invention to reduce the number of wires to solve the problem of out-of-timing pulses due to differences in wire length and to emit light pulses with a shorter emission time at a high transfer rate. It is another object of the present invention to provide a semiconductor laser drive circuit for switching between a low-noise laser and a high-output laser.